This invention relates to hydrocarbon conversion catalysts, and particularly to those utilized to catalyze the reaction of hydrogen with organic compounds containing nitrogen and/or sulfur so as to yield a denitrogenated and/or desulfurized product. More particularly, the invention relates to a process for removing compounds containing silicon from hydrocarbon streams and is particularly concerned with a process for removing organosilicon compounds from reformer feedstocks to prevent silicon poisoning of the reformer catalyst.
In the refining of liquid hydrocarbons derived from mineral oilos and other sources, ti is often desirable to subject the liquid hydrocarbon or fraction thereof to hydrotreating. Hydrotreating is a refining process wherein liquid hydrocabons are reacted with hydrogen. Hydrotreating is often employed to reduce the concentration of olefins and oxygen in hydrocarbons. Hydrotreating is most commonly employed, however, to reduce the concentration of nitrogen and/or sulfur in hydrocarbon-containing feedstocks. Reducing the concentration of nitrogen and sulfur produces a product hydrocarbon which, when eventually combusted, results in reduced air pollutants of the forms NO.sub.x and SO.sub.x. Reducing the concentration of nitrogen is also desirable to protect other refining processes, such as hydrocracking, which employ catalysts which deactivate in the presence of nitrogen.
In general, the hydrotreating of a nitrogen and/or sulfur-containing feedstocks is carried out by contacting the feedstock with hydrogen at elevated temperatures and pressures and in the presence of a suitable catalyst so as to convert the nitrogen to ammonia and the sulfur to hydrogen sulfide.
A typical hydrotreating catalyst comprises particles containing a Group VIII active metal component and a Group VIB active metal component supported on a refractory oxide such as alumina. Phosphorus components are commonly incorporated into the catalyst to improve its activity by increasing its acidity. One catalyst which has been successfully employed on a commercial basis consists essentially of molybdenum, nickel, and phosphorus components supported on gamma alumina. Improved activities and stabilities of such catalysts are continuously being sought. The higher the activity of the catalyst, the lower the reactor temperature required to obtain a product of given contaminant (sulfur, nitrogen, etc.) content from the feedstock. The lower the reaction temperature, the lower the expense of hydrotreating a given unit of feedstock due to savings in process heat requirement. Furthermore, hydrotreating at a lower reaction temperature usually extends the life of the catalyst, i.e., increases catalyst stability, assuming, of course, that all other process parameters are held constant.
Catalytic reforming is a conventional refining process which is utilized for such purposes as dehydrogenation, hydrogenation, cyclization, dehydrocyclization, isomerization and dehydroisomerization of selected hydrocarbons. Catalytic reforming is normally utilized to upgrade straight run or cracked naphtha feedstocks by increasing the octane number of the feedstock's gasoline fraction. In a typical reforming process in which a straight run or cracked naphtha is upgraded, the feedstock is contacted with a catalyst comprising a noble metal on alumina. The conditions utilized in the reforming process will vary depending upon such factors as the type of feed being processed and the desired increase in octane level.
Reforming catalysts, particularly those containing platinum, and most particularly those comprising platinum, rhenium and chlorine, are poisoned or deactivated rapidly in the presence of sulfur components. Thus, to achieve maximum run lengths and increase process efficiency, it is necessary to reduce the sulfur content of reformer feedstocks as low as possible.
In addition to being highly sensitive to sulfur components, reforming catalysts are also poisoned by compounds containing silicon. One common source of hydrocarbon streams containing silicon compounds is the delayed coking unit utilized in many petroleum refineries. Such a unit is used to convert residual oils into more valuable products. The overhead vapors from the coking drum, which is part of the delayed coking unit, are normally fractionated into various cuts including a gasoline boiling range stream commonly referred to as coker gasoline or coker naphtha. This stream generally possesses a low octane number and is therefore unsuitable for use as automotive fuel without upgrading. Thus, it has become common practice to increase the octane number of coker gasoline by subjecting it to catalytic reforming. The coker gasoline will not only contain sulfur (and nitrogen) compounds but, quite frequently, will contain organosilicon components derived from silicon defoamers, such as polydimethyl siloxanes, added in the delayed coking process to prevent foaming.
It is desirable to remove both sulfur compounds and silicon compounds from coker gasoline and other hydrocarbon streams that are to be processed in catalytic reformers. If a stream containing both sulfur and silicon compounds is subjected to catalytic hydrodesulfurization or hydrotreating, the sulfur will not only be removed by conversion to hydrogen sulfide (and the nitrogen to ammonia) but the silicon compounds will deposit on the catalyst. The deposited organosilicon components will have a deleterious effect on the hydrotreating catalyst, tending to deactivate it and decrease its stability.
Accordingly, the present invention provides a process for removing silicon components from hydrocarbon feedstreams during catalytic hydrodesulfurization or hydrotreating. The invention further provides a process for removing silicon components from feedstreams which have previously been subjected to delayed coking. Alternatively, the invention provides a process which can be used to simultaneously remove sulfur (and/or nitrogen) and silicon components from hydrocarbon-containing feedstreams such as reformer feeds.